1. Field
Embodiments of the present disclosure relate to systems and methods for obtaining diagnostic information related to the health of sensors in a complete meter proving system including a bi-directional prover in a portion of a pipe for oil-related services.
2. Description of Related Art
Background
It is important in the oil and gas industry to accurately measure the amount of hydrocarbons being transferred in pipeline systems because the material is being bought and sold based upon quantity. When crude oil and natural gas are removed from the ground, they are transported via pipelines from one location to another and the amount of fluid being transferred is measure by a flowmeter. A flowmeter measures the volume of a liquid being sent through a pipeline at various points in the pipeline system where the hydrocarbons “change hands.” Flowmeters generally provide pulses to a computer system, which counts and determines the volume based on the number of pulses counted. Examples of flowmeters include turbine meters, vortex meters, ultrasonic meters, positive displacement meters, or any kind of meters that are used in pipeline systems to determine the volume of the liquid.
As noted above, it is important to measure liquid flow volume accurately because ownership of the commodity changes at various points. The accuracy of the flowmeter can be affected by the variation in the characteristics of fluid that is measured. Also, changes in the operating process conditions such as temperature fluctuations and the life cycle of the meter can effect the accuracy of the flowmeter.
Provers are used in pipeline systems transporting various liquid hydrocarbons to accurately calibrate the volume readings of the flowmeter. Provers contain detector sensors that must be constantly checked for accuracy and health to ensure that the prover is accurate and functioning properly. One type of prover is the sphere prover, but other types such as a piston are also known and can be used in the present invention. Sphere provers are used to check the accuracy of the flowmeter and do so by calculating a meter factor for each section of the prover. The prover has a known volume which is calibrated to known set standards of accuracy. The known volume is the amount of fluid that is displaced by the prover sphere as it passes through the pipe section between two detector switches. The base prover volume of liquid that passes through the prover is compared to the meter reading and if the comparison is within a prescribed threshold, then the meter is accurate within the required tolerance levels. If the volume through the prover is significantly different from the meter, then a meter factor can be used to adjust the meter reading to accurately reflect the amount of liquid being pumped through the pipe. The meter factor for a particular section of pipe is found by taking the known base prover volume divided by the flowmeter amount.
Further, provers can be bi-directional or uni-directional. Uni-directional provers contain a displacer, which moves inside the prover pipe section and can be shaped based upon the shape of the pipe. Displacers are propelled by the force of liquid behind the prover, travel in one direction between the detectors and require a displacer handling device where the displacer is moved back to the first sensor in order to recalibrate the meter. Bi-directional provers use a single displacer that is cycled back and forth within a calibrated meter prover pipe or barrel with a pair of detectors at either end. The fluid, from a main pipe, flows in a first direction usually directed by a four way valve pushing the displacer, which is usually a sphere, through the prover. After the displacer gets through the pipe section the four-way valve changes position and the displacer is moved back through the prover to its original position. Sometimes the four-way valve is controlled by a computer, which provides an actuating command for the valve. A pass can refer to the movement of the displacer from one side to the other, while a trial run is a combination of two passes resulting in the displacer moving from the original position to the other side and then back to the original position.
In general, meter proving systems in the U.S. and internationally are governed by the American Petroleum Institute Manual of Petroelum Measurement Standards, Chapter 4-Proving Systems, Section 2, Displacement Provers, Third Edition, September 2003, Reaffirmed March 2011 (API MPMS Chap 4.2). Meter proving using a bi-directional sphere prover is approved by the API standard and the present invention is related to a bi-directional sphere prover.
A typical sphere prover is a section of pipe that is attached to a line delivering hydrocarbon liquid. A sphere travels through the section of pipe actuating a detector at each end of the calibrated section. There is usually a prover computer that activates and totals the flow measured by the flowmeter when the sphere pass through detecting point at each end of the calibration section and stops totalizing when the sphere reaches the other ends of the calibration section. The passing of the sphere through the two detector points is timed, and in effect it is the totalized flowmeter reading for the time required for the sphere to travel between the detecting points. This number is compared with the known volume of the metering pipe between the detecting points to provide an accurate calibration and meter factor.
When meters cannot be proved, there is a potential for error in the flowmeter readings because the flowmeter is not properly adjusted by a meter factor or it is unclear if there needs to be an adjustment of the meter based on the meter factor. The flowmeter in that case reports a reading that is not the accurate total of the fluid that has passed through the pipe. Meter factors are calculated using the difference between the reference base prover volume and the number of pulses counted from the flowmeter while the liquid hydrocarbons are moving through the prover section of pipe. Meters cannot be proved when the prover is inaccurate or down for repair because one of the detectors in the prover is not working properly. Enabling provers to be repaired and monitored quickly will increase the utilization of the prover. Further, these systems need to be continuously monitored because the base prover volume can change due to the buildup of sticky and waxy material inside the prover calibration section.
Normally, provers only have two detector switches according to API 4.2 paragraphs 2.9 and 3.8. The calibrated volume between the two detector switches is defined as Base Prover Volume (BPV) which is used to calculate the meter factor. More switches can be used if more than one calibrated volume is required on the same prover or the detectors can be used to signal that the displacer has entered the sphere resting chamber. Once the sphere has passed through the entire section of the tubes, or sections, then the sphere is reversed and another set of measurements is taken to double check the volume. Usually there are detectors at either end of the prover pipe and four-way valves are operated after the sphere has gone one direction to push it back the other direction.
Flowmeters constantly undergo calibration after each fixed period to check whether or not the metering precision is within a fixed range. Usually a reference volume is used for the calibration. Further, in some systems multiple pairs of detectors to record multiple calibrated prover volumes with a single pass of the displacer/sphere. A computer is usually used to record these volumes, and more than two pairs of sensors can be used. The meter volume is usually determined by pulses that accumulate while the displacer is being moved through the prover and then a flow computer is used to compare the amount of pulses and volume that is arrived at through a pulse-per-unit volume and flow rate to the prover volume.
A pulse is proportional to flow volume and pulse rate is proportional to flow rate. There is a factor that correlates the pulse per unit volume and that can be used to determine the flow rate and volume of the meter. Finally a meter correction factor is generated based upon the discrepancy between the known prover volume and the meter readings based on the pulses. This is applied to the meter and the processor in the flow computer can look to the repeatability of the meter factor to make sure it is within an acceptable range.
Some detector switches can be monitored based upon their output readings when the displacer passes by them. For instance the rising and falling edge of the pulse received from the detector can be analyzed to determine the health of the detector and compared with snapshots of the performance of the detector from the past to determine if the detector switches are performing adequately. In some meters, acoustic signals are sent through the liquid and the transducers used to send these signals may degrade over time. In order to monitor these detectors, the signal to noise ratio, signal amplitude or noise amplitude are used to indicate the overall health of the particular detector being pulsed. Another method that is commonly used in the field, is that a calibration table is built up during a learning period that associates each sensor reading with a flow value. When the sensor reading is off by a threshold amount in the operating period, then the sensor value is replaced from the calibration table. This method is used when meter servicing is very difficult. Detectors can also include inductive or mechanical protrusion detectors.
All of the techniques discussed to derive prover diagnostic information do not take into account historical trends in meter factors and meter factor ratios to determine if the switches are in good health.